Effects of Flapping Frequency on the Aerodynamic Performance of the Elliptical Tandem Flapping Wings

The present numerical study investigated the effects of the flapping frequency on the aerodynamic performance of two-dimensional tandem elliptical flapping wings in the presence of incoming flow at a Reynolds number of 5000. We have considered identical front and rear wings, keeping their aspect ratio fixed at 8. The spacing between the wings is fixed as 1c, where c is the wing chord length. The non-dimensional flapping frequency or Strouhal number, St, is varied from 0.1 to 0.7 to investigate the thrust performance of the tandem flapping wings at various effective angle of attack amplitudes (α0 = 10°, 15°, and 20°). It is noticed that upon increasing the St, similar to an isolated wing, the time-averaged thrust coefficient of the front and rear wing increases up to the critical St (Stcr), and beyond that, it deteriorates significantly for all the α0 values. Moreover, the thrust performance of the rear wing is superior to the front wing and isolated wing in St < Stcr. However, the Stcr of the rear wing is found less compared to the front and isolated wing. The combined time-averaged thrust of the tandem flapping wings is always observed to be higher than that of the isolated flapping wing. Additionally, we have thoroughly analyzed the flow physics associated with the aerodynamic performance of the tandem flapping wings.